1. Field of the Invention
This invention relates to aftertreatment devices for treating engine exhaust streams, and more particularly relates to detecting the occurrence of temperature threshold events in aftertreatment devices.
2. Description of the Related Art
Emissions regulations for internal combustion engines have changed rapidly in recent years. To meet the new regulations, many engine manufacturers have had to install aftertreatment devices to reduce emissions in the exhaust gases, or to condition the exhaust gases to assist other aftertreatment devices. For example, particulate filters remove soot from the exhaust gases of a diesel engine, and diesel oxidation catalysts are sometimes used to generate temperature in the exhaust gas to assist a particulate filter in oxidizing the soot off of the filter.
Most aftertreatment devices experience thermal cycles during the operations of the engine. The thermal cycles may be intentional, for example during the removal of soot from a particulate filter, or unintentional such as when the engine experiences large changes in the required workload for the engine. Each thermal cycle induces a temperature gradient within the device. The temperature gradient within the device may cause stresses and over time can cause the aftertreatment device to fail. In general, the higher the maximum temperature experienced within the aftertreatment device, the larger the thermal gradient within the aftertreatment device. A high temperature can also cause stress and/or failure of an aftertreatment device independent of the temperature gradient induced in the device.
A stress related failure within an aftertreatment device, such as a crack in the wall of the aftertreatment device, can be particularly difficult to detect. There are no direct measurements routinely used in real-time for applications to detect such failures. Even when an aftertreatment device is being serviced, it is difficult for a service technician to detect such a failure even if the technician has a reason to look for it.
The aftertreatment device typically comprises a core—such as cordierite or silicon carbide honeycomb structure—wrapped in an insulating material that fixes the core in place, and the whole device is typically covered by a sheet metal and/or stainless steel shell or “can.” A stress failure on a device occurs in the core, typically as radial cracking around the surface of the core, and is not visible to a technician merely handling the device. Therefore, the current detection failure schemes rely on either ultrasound or special visual inspection to determine whether an aftertreatment component has failed.
Ultrasound detection schemes are problematic because of the intentional porous nature of the aftertreatment devices, and the gaps in the surrounding insulating material. The ultrasound frequency must be so low (causing a low resolution image), and the aftertreatment devices are so poorly configured for ultrasound analysis, that often only the most catastrophic failures can be detected. However, some aftertreatment devices are no longer design compliant—which can mean regulatory emissions thresholds are not being met—with only a few moderate cracks around the device.
Special visual inspections require optic tools allowing the technician to view the interior of channels within the aftertreatment device. The channels of the device may be packed with soot and/or debris, rendering the inspection difficult or impossible. A minimal check of the device may require checking hundreds of channels around the perimeter of an aftertreatment device by repeatedly inserting a tool designed to go into small diameter channels which are at a packing density of 200-300 cells per square inch. The inspection procedure can damage the aftertreatment device, and is time consuming and costly under the best of circumstances.
Even where physical device failure of the aftertreatment device can be detected, high temperatures within the aftertreatment device can cause excessive degradation short of physical device failure. For example, an aftertreatment device may be expected to crack at 950 degrees C., but experience severe catalyst deactivation at 850 degrees C. with no physical indications of degradation. Catalyst degradation may induce additional stress on the device—for example increasing the average temperature at which soot oxidation can occur, and catalyst degradation may cause emissions increases. An aftertreatment device may be experiencing increased emissions without being detected.
A detection of the true temperature within the aftertreatment device is currently beyond the current technology at commercially reasonable prices. Current aftertreatment systems place a temperature sensing device—usually a thermistor and/or a thermocouple—just upstream and/or downstream of the aftertreatment device. The temperature within the aftertreatment device is often estimated as a function of these temperatures—for example a weighted average of the temperatures, or a thermal model based on the temperatures and estimated hydrocarbon or soot burning rates within the aftertreatment device plus estimated heat transfer effects. While the current temperature estimates are acceptable for certain estimates such as determining soot oxidation rates in steady state operation, the current temperature estimates do not estimate peak temperature events in transients very well. For example, a temperature spike may occur within the aftertreatment device, but the delay on the temperature sensing devices may cause the temperature sensing device to miss the highest portions of the spike and show temperatures 100 deg C. or more lower than the actual temperature event experienced within the aftertreatment device.
These limitations in the current technology introduce the risks attendant with aftertreatment devices with hidden defects. For example, a service company may clean aftertreatment devices and swap them out for a dirty aftertreatment device in a customer vehicle. Under the current state of technology, there is a significant risk that one of the swapped aftertreatment devices may have a stress failure or degraded catalyst, penalizing either the customer or the service company according to which device has failed.